Squaraine dye

As squaraines is known since 1980, a class of intensive dyes whose basis are the squaric acid or esters thereof. They are therefore sometimes referred to in the German-speaking area as squaric acid dyes, older - no longer used - designations are Tetracyclotrimethine, cyclobutenediylium dyes or substituted Oxocyclobutenolate.

General

Due to their unusual structural, electronic and photophysical properties of squaraine dyes are not only of scientific interest. They show a high thermal and photochemical stability and were therefore used shortly after their discovery as sensitizers for inorganic photoconductor. Some time later, squaraines were used as chemical sensors, and as a xerographic photoreceptor. Today we are particularly interested in integrating Squarainstrukturen in conjugated polymers. This should open the way for new management or other photoconductive materials.

Definition and discovery

The squaraines were discovered in the mid 1960s by two research groups simultaneously and independently. Your name squaraine is a combination of squaric acid, the English term for the squaric acid, and betaine, which zwitterionic molecules with negative and positive charge center are referred to.

The squaraines are always 1.3 dicondensation of squaric acid with nucleophilic compounds and therefore all contain a cyclobutenediylium -1 ,3- diolate unit. As nucleophiles, for example, can primary and secondary amines, electron-rich aromatic and heteroaromatic compounds serve.

This basic structure of squaraines explains their intense color. Formally speaking, corresponds to the central squaraine four ring a Hückelaromaten with two π - electrons. Thus, an electron conjugation over the entire molecule is planar built across possible. The two electron-rich aryl radicals in mainstream push electrons in the electron-poor four-membered ring. The result is a donor-acceptor -donor system with strong intramolecular charge transfer effects.

Depending on the structure of the compounds in symmetrical and unsymmetrical squaraines can be divided. As unbalanced squaraines is called squaric acid derivatives in the 1 - carry different substituents and 3- position, whereas symmetric squaraines bear the same substituents.

Synthesis of squaraines

In the literature different ways of Squarainsynthese be discussed. The three most important are presented here.

Direct aromatic- squaric acid condensation

In this case, by heating of squaric acid in an alcohol (usually butanol) is formed a squaric acid monoesters. The aromatic nucleophile attacks at the vinylogous ester carbon and substituted alkoxy group with elimination of the corresponding alcohol. At this 1:1 adduct then another nucleophile in the trans position can be added to the first nucleophile to the carbonyl carbon of the four-membered ring. The major disadvantage of this route is the limited synthesis diversity that arises from the need for the use of electron-rich, sterically less demanding aryls.

Ketendimerisierung

The second way is based on a dimerization of ketenes. These are generated to in situ by dehydrochlorination of phenylacetyl chlorides. The resulting ketenes react in a [2 2 ] cycloaddition to a Dihydrosquarain from which we obtain the desired squaraine by oxidation. The advantage of this route is that even electro poorer aryl systems can be used. As possible, for example, the synthesis of the bis (4- methoxyphenyl) squarains only in this way.

Synthesis via Halbsquarainaddukte

Halbsquaraine (ie 4-aryl -1-hydroxy -cyclobutene -3 ,4 -diones ) can be represented in three ways; the desired squaraine then obtained by condensation with activated aromatics.

Applications of squaraines

The first squaraines were discovered in 1965 by Treibs and Jacob. You condensed squaric acid with substituted pyrroles and received as a result of deeply colored, highly thermally stable compounds. This was followed by squaraines on the basis of N, N -dialkyl- aniline derivatives. This resultant bis (N, N- dialkylaminophenyls ) squaraines make today an important class dar. followed by condensation with pyrazolone, benzothiophene and barbituric acid derivatives squaraines recovered.

Originally put a squaraines as sensitizers for ZnO photoconductors, in the 1970s scientists studied their applications in the field of solar technology. As squaraines show an intense absorption in the near infrared region, the use in optical storage media, or as laser dyes is also possible.

In the 1990s, the research was taken up to squaraine synthesis due to their attractive photophysical properties again. By the introduction of more highly substituted aromatic compounds with additional hypsochromic and bathochromic groups in the ortho and / or meta- position of the properties such as solubility, crystal formation or light absorption can be controlled. The goal is to improve the technical photoconductivity and other xerographic properties.

Squaraines today mainly as fluorescent dyes in the DNA sequence analysis, as well as materials for non- linear optics (NLO ) may be used. Since the late 1990s, the research on the synthesis of oligo-and Polysquarainen and chiral squaraines focused.